A.—­It is not indispensable that
the whole tire should be of steel; but a dovetail
groove, turned out of the tire at the place where it
bears most on the rail, and fitted with a band of
steel, will suffice. This band may be put in
in pieces, and the expedient appears to be the best
way of repairing a worn tire; but particular care
must be taken to attach these pieces very securely
to the tire by rivets, else in the rapid revolution
of the wheel the steel may be thrown out by the centrifugal
force. In aid of such attachment the steel, after
being introduced, is well hammered, which expands
it sideways until it fills the dovetail groove.

531. Q.—­Is any arrangement adopted
to facilitate the passage of the locomotive round
curves?

A.—­The tire is turned somewhat conical,
to facilitate the passage of the engine round curves—­the
diameter of the outer wheel being virtually increased
by the centrifugal force of the engine, and that of
the inner wheel being correspondingly diminished,
whereby the curve is passed without the resistance
which would otherwise arise from the inequality of
the spaces passed over by wheels of the same diameter
fixed upon the same axle. The rails, moreover,
are not set quite upright, but are slightly inclined
inward, in consequence of which the wheels must be
either conical or slightly dished, to bear fairly
upon the rails. One benefit of inclining the
rails in this way, and coning the tires, is that the
flange of the wheels is less liable to bear against
the sides of the rail, and with the same view the
flanges of all the wheels are made with large fillets
in the corners. Wheels have been placed loose
upon the axle, but they have less stability, and are
not now much used. Nevertheless this plan appears
to be a good one if properly worked out.

532. Q.—­Are any precautions taken
to prevent engines from being thrown off the rails
by obstructions left upon the line?

A.—­In most engines a bar is strongly
attached to the front of the carriage on each side,
and projects perpendicularly downward to within a
short distance of the rail, to clear away stones or
other obstructions that might occasion accidents if
the engine ran over them.

CHAPTER IX.

STEAM NAVIGATION.

* * * *
*

RESISTANCE OF VESSELS IN WATER.

533. Q.—­How do you determine the
resistance encountered by a vessel moving in water?

A.—­The resistance experienced by
vessels moving in water varies as the square of the
velocity of their motion, or nearly so; and the power
necessary to impart an increased velocity varies nearly
as the cube of such increased velocity. To double
the velocity of a steam vessel, therefore, will require
four times the amount of tractive force, and as that
quadrupled force must act through twice the distance
in the same time, an engine capable of exerting eight
times the original power will be required.[1]